No proof or scientific studies to back me up, but I think the reason a lot of guys used to run those venturis is for a more practical reason? It's to reduce the goop buildup due to a piston ported engine's tendency to blow back through the open intake port just before it closes. If it were not for the prop scattering it, on an engine running without a venturi, you would be able to see a fog or haze consisting of fuel and oil droplets just outside the carb throat when it's running at speed. Adding a venturi extends the engine's intake tract in an attempt to contain or capture that "fog" so the prop wash doesn't blow it down the side of the plane? -Al